The ceramic heater comprising a resistance heating element of high-melting metal as embedded between a core and an insulation sheet covering the core is in widespread use as a heating means for the automotive oxygen sensor, glow system, etc. or as a heat source for devices for gassification of petroleum oil, such as a heater for use in semiconductor heating or an oil fan heater.
FIG. 5(a) is a perspective view showing a typical ceramic heater of this type schematically and FIG. 5(b) is a sectional view taken along the line A--A of FIG. 5(a).
This ceramic heater comprises a cylindrical core 31, an insulation sheet 32 wrapped around said core 31 with an adhesive layer 37 interposed, and a resistance heating element 33 embedded between said core and insulation sheet, with terminal portions of said resistance heating element 33 being connected to external terminals 34 disposed externally of said insulation sheet 32 and lead wires 36 being connected to said external terminals 34, respectively.
As shown in FIG. 5(b), each terminal portion of said resistance heating element 33 is connected to the corresponding external terminal 34 via a plated-through hole 35 provided under said external terminal 34 in the insulation sheet 32. In this arrangement, as an electric current is applied between the external terminals 34 through the lead wires 36, the resistance heating element 33 generates heat and thereby functions as a heater.
The insulation sheet 32 of said ceramic heater generally comprises Al.sub.2 O.sub.3 supplemented with, as sintering aids, SiO.sub.2, MgO, CaO, etc., and the SiO.sub.2, MgO, etc. are segregated as glass phases in the grain boundaries of alumina ceramics.
When a ceramic heater of this type is used as a heat source for the oxygen sensor of an automobile, for instance, a 12V direct current is applied between the terminals 34 of the ceramic heater, whereupon the resistance heating element 33 of the heater reaches to a high temperature of about 1000 to 1100.degree. C. at the maximum.
Since the Mg and Ca in the insulation sheet 32 are present chiefly as glass phases in the grain boundaries, prolonged operation of the heater under such high-temperature DC conditions results in attraction of Mg.sup.2+ and Ca.sup.2+ in glass phases toward the negative pole so that the so-called migration, i.e. a shift of said metal ions toward the negative terminal, takes place. As this migration occurs, voids are formed in the grain boundaries of the alumina ceramics.
As the amount of voids in the alumina ceramics increases, the resistance heating element embedded beneath the insulation layer comes into contact with the air infiltrating into the voids, resulting in a progress in oxidation of the resistance heating element, with the result that not only is the resistance value of the heating element increased gradually but the resistance heating element as such expands due to oxidation. As a result, the heating temperature of the resistance heating element varies and the heating element becomes liable to be destroyed and, in extreme cases, develops a disconnection trouble.